Debug mode about nixpkgs-fmt

[matklad]

Yeah, I also find it quite hard to debug interactions between the rules. Though, I am not sure if just somehow showing matching rules would help: it seems like it would produce way to much information to be useful.

At the moment, I just comment out some rules to minimize the example. I imagine that could be automated: formatting could print a series of diffs, starting with "zero rules applied", and ending with "all rules applied". Still not sure if that would be more effective than just manually commenting out specifc things

[zimbatm]

The difference would mainly be that as a user it would be possible to understand what is going on without toughing the code (assuming that the output is helpful). I think rules would also have to have names attached to them so they could be distinguished one from another.

Brainstorming a bit more, another approach might be to attach some assertions onto a node, asserting that rule X is going to be applied to it.

[matklad]

This is implemented as --explain flag in #79!